The present invention relates to electronically scanned type radar antenna systems comprised of a plurality of antenna elements, the signals of which are phase controlled with respect to each other to steer the beam of the antenna in a desired direction, and more particularly to a phase shift controller associated with each of the elements of the antenna for individually computing a phase shift value sequence to govern the phase shift of the corresponding antenna elements in accordance with a predetermined time pattern to increase the speed of antenna beam steering.
Generally electronic scanning type radar antennas are comprised of a plurality of elements which are phase shifted with respect to each other to steer the antenna radar beam. In digitally controlled antenna phased arrays similar to those disclosed in U.S. Pat. Nos. 3,482,244; 3,646,558; and 3,680,109, the phase of each element of the antenna array is controlled by a phase shifter which is usually individually governed by a phase shift controller. Each phase shift controller is provided with a phase shift value which is normally computed in a digital computing device. As a new set of phase shift values associated with the array are loaded into their respective phase shift controllers, the antenna beam is correspondingly steered to a new position. Accordingly, then if a plurality of array sets of phase shift values are computed by the digital computing device in accordance with some predetermined pattern and each new array set is sequentially loaded into the phase shift controllers of the antenna array at some predetermined loading rate, the beam of the antenna may be incrementally steered to scan a portion of space as directed by the precomputed pattern of phase shift value array sets. It is understood that the rate at which the beam is steered across the space is limited primarily by the computation and register loading times of the new phase shift values. In most cases, it is preferred to keep the beam scan increment small to achieve adequate resolution of the portion of space being scanned, however, this tends to increase the frequency of computation and loading combinations for generating each new array set of phase shift values. This may present somewhat of a dilemma should high speed beam steering be additionally specified.
With conventional radar sets which comprise both the transmitter and receiver antennas as an integral unit, the beam of both the transmitter and receiver are normally pointed in the same direction by the very nature of the design. In these conventional radar sets, there is generally no apparent requirement for high speed beam steering and the methods utilized for phase shifting the elements of a phased array antenna, typical of those disclosed in the aforementioned U.S. Pat. Nos. 3,482,244; 3,646,558; and 3,680,109 and that which has been described hereinabove, have been reasonably sufficient in most cases.
Recently, unconventional radar sets, such as a bistatic radar system, have been found to offer certain anti-jamming protection features over conventional radar sets against enemy radar jamming measures. Bistatic radar systems such as the one disclosed in U.S. Pat. No. 3,842,417 utilize different antennas for transmitting and receiving radar energy. Generally, when a transmitter of a conventional radar emits radiation which is possibly detected by the enemy, jamming energies may be transmitted by the enemy in the direction of the transmitter to confuse reception. However, in bistatic radar systems, the receiver is not part of the radar transmitter, but located away from the transmitter and is not influenced by the enemy jamming signals, thereby providing the anti-jamming protection. The bistatic radar receiver can receive radar echo signals from potential targets as long as it is capable of "pulse chasing" the radar pulsed transmissions to establish a target location and track the detected target thereafter. In a typical bistatic radar system, pulsed energy is radiated from the transmitter in a pencil beam at a predetermined time and in a prespecified direction in space. The pulsed energy follows the prespecified beam direction at the speed of light. To "pulse chase", the beam of the receiver of the bistatic radar system must be directed to follow the pulse of energy from the transmitter as it is radiated out in space in the pencil beam at the speed of light so that any reflected energy from a target located within the pencil beam of the transmitter will be received within the beam of the receiver. For this reason, the beam scanning speed of a bistatic radar receiver, in some cases, must be maintained close to the speed of light.
Bistatic radar receivers are usually electronically scanned phased arrays similar to those disclosed in U.S. Pat. Nos. 3,825,928 and 3,978,482. The limiting factor in the receiver's capability of "pulse chasing" is primarily the speed at which the digital computing device can calculate each new array set of phase shifting values for the phase shift controllers which govern the phase of the elements of the receiver array to incrementally set the next beam scan angle or angle direction jump. Another time problem associated with "pulse chasing" is related to the loading of all the registers of the phase shift controllers with the next array set of phase shifting values prior to incrementing to the next angle in the beam scan of the receiver. Attempts to achieve incremental phase stepping at megahertz rates have been expensive to achieve using the conventional beam steering techniques described supra. The present invention disclosed hereinbelow offers unconventional techniques for calculating the phase shifting values of each of the elements of the receiver's phase array to provide low cost, very rapid non-uniform beam scan capability for such high speed beam steering applications like "pulse chasing" for target location detection in bistatic radar sets, for example.